The present invention relates to a color video printer, and more particularly to a printing method for the high-speed printing of a color video printer using a frame-sequential method, and a printing apparatus suitable for employing the method.
Color video printers output a hard copy of an input still image signal. In printers using a sublimation thermal transfer system, current corresponding to a video signal to be written is supplied to a thermal printing head (TPH: a write electrode composed of a series of heat emitting devices), and dyes adhered to a transfer film disposed adjacent thereto are allowed to sublimate by heat generated from the TPH, so that image information is written on a sheet of paper.
The printers using the sublimation thermal transfer system separate a video signal of one frame into a color signal having RGB components, and perform the printing by sequential frames of each color component.
FIG. 1 is a block diagram showing a conventional color video printer. The color video printer shown in FIG. 1 carries out the printing by selectively receiving a composite video signal from a display (e.g., a TV receiver or video recorder) and a super video signal from a graphic apparatus (e.g., a computer).
A signal in the form of a composite video signal is supplied to a luma/chroma signal (Y/C) separator 1. The output from Y/C separator 1 is supplied to one input of an input selector 2. In the meantime, the other input of input selector 2 is supplied with a signal in the form of a super video signal.
The signal selected in input selector 2 is supplied to a color difference signal decoder 3. The color difference signal in color difference signal decoder 3 is supplied to a still/moving picture selecting unit comprising a first selection switch 4a and a second selection switch 4b. The color difference signal output from one output of first selection switch 4a is input to a still picture memory 5 whose output is supplied to one input of second selection switch 4b. Meanwhile, the color difference signal from the other output of first selection switch 4a is directly supplied to the other input of second selection switch 4b. The color difference signal in second selection switch 4b is supplied to an RGB decoder 6. The color signal output from RGB decoder 6 is simultaneously supplied to an encoder 7 and a printing unit 10. The composite video signal from encoder 7 is monitored via a display 8.
Meanwhile, the color signal supplied to printing unit 10 is hard-copied via a TPH 20. Hereinbelow, the construction of printing unit 10 will be described in detail.
The, color signal input to printing unit 10 is first supplied to an RGB selector 11. The color signal selected in RGB selector 11 is supplied to an analog-to-digital (A/D) converter 12. The signal digitized in A/D converter 12 is supplied via n bit lines to an input switch 14. The signal from input switch 14 is supplied to a line memory 16. The signal from line memory 16 is supplied to an output switch 18. The signal from output switch 18 is supplied to an intermediate gradation converter (IGC) 19. The pulse-width-modulated signal from IGC 19 is supplied to TPH 20 which then carries out the printing. A sampling pulse generator 13 generates a sampling pulse required in A/D converter 12 and a line memory controller 17 which, using the sampling pulse, produces an address and control signal required in line memory 16. An I/O switching controller 15 generates a signal for controlling both input and output switches 14 and 18.
Now, the operation of the printer having the above-described construction will be described. The "composite video signal separated into luma/chroma" or "super video" signal selected corresponding to a composite/super selection signal in input selector 2 is supplied to color difference signal decoder 3. The color difference signal having R-Y, B-Y and Y components in color difference signal decoder 3 is supplied to the still/moving picture selector unit (4a and 4b). In conjunction with the still/moving picture selector unit, still picture memory 5 is provided to store a video signal in frames, for the case of the input signal being a moving image signal.
When the moving picture is selected by means of a still/moving picture selection signal, first selection switch 4a is controlled to output the input signal to a first output, and second selection switch 4b is controlled to output the signal input to its first input. By this operation, a video signal of one frame in the moving video signal input to the color video printer is separated, to thereby be stored in still picture memory 5; otherwise, the still picture signal stored in still picture memory 5 is output to be printed.
On the other hand, when the still picture is selected, first selection switch 4a is controlled to output the input signal to its second output, and second selection switch 4b is controlled to output the signal input to its second input. Accordingly, the still picture signal input to the color video printer is thus printed.
Since the moving picture signal input to the color video printer is composed of a video signal having several frames, one frame is separated to thereby be stored as a still picture. In this state, the video signal input to still picture memory 5 is digitized in an analog-to-digital (A/D) converter 5a and then stored in a memory 5b. Therefore, when the still image signal stored in memory 5b is to be printed, it is first read out and converted into an analog signal via a digital-to-analog (D/A) converter 5d, and then the analog signal is output. Memory controller 5c generates an address and control signal for memory 56.
The color difference signal from second selection switch 4b is converted into R, G and B signals suitable for use in printing unit 10 by means of RGB decoder 6, and then is supplied to RGB selector 11 of printing unit 10. R, G or B signal selected in RGB selector 11 is supplied to the cyan/magenta/yellow transfer film, and then hard-copied onto the sheet of paper. In accomplishing such a hard copy, the B signal is first selected to print the B component of the still picture, followed by the G signal being selected to print the G component, and then the R signal is selected to print the R component. This printing method is called the "frame-sequential" method because a still picture's RGB components are sequentially printed in frame units.
Next, the printing of the selected R, G or B signal will be described in detail. The signal selected in RGB selector 11 is digitized in A/D converter 12, and then supplied to input switch 14. In performing the frame-sequential printing of one frame of a still picture, the color video printer having the structure shown in FIG. 1 sequentially prints the pixels in the vertical direction of the pixel array constituting one frame, from left to right by columns. By this operation, A/D converter 12 must perform the sequential sampling of the vertical lines of the pixel array constituting one frame, from left to right. The sampling pulse corresponding to this sampling is produced in sampling pulse generator 13.
The digitized pixel signal from input switch 14 is stored in line memory 16 to be output to IGC 19 via output switch 18, thereby being pulse-width-modulated. The pulse-width-modulated signal is supplied to TPH 20 so that the printing can be executed.
Line, memory 16 for the reading and writing of pixels in the column direction which are digitized in A/D converter 12, includes two line memories (LM) for performing high-speed printing, so that the read/write operations are carded out by the complimentary operation of first and second line memories 16a and 16b. The control signal needed in input and output switches 14 and 18 is supplied from I/O switching controller 15, and the address and control signal for read/write operation of line memory 16 is supplied from line memory controller 17.
During printing by printing unit 10, the picture being printed is monitored via encoder 7 and display 8.
FIG. 2 shows the relation between the pixel array and the line memory in one frame. Here, the oblique solid lines indicate the horizontal scanning lines in an odd field, and the oblique dotted lines indicate the horizontal scanning lines in an even field. In the NTSC system, each field has 262.5 horizontal scanning lines which are interleaved with each other.
For convenience of explanation, the first horizontal line of a odd field is called a first scanning line, the first horizontal scanning of an even field is a second scanning line, the second horizontal line of an odd field is a third scanning line, and the second scanning of an even field is a fourth scanning line. Accordingly, in FIG. 2, the odd field consists of the odd-numbered horizontal lines, and the even field consists of the even-numbered horizontal lines.
The, pixel array of one frame is formed by a number of pixels X per horizontal scanning line and a number of pixels Y in the vertical direction. For example, in the NTSC system, X is 600 and Y is 525 which is the same as the number of horizontal scanning lines. Also, assuming that an interval of the horizontal scanning line from the first pixel to the last pixel is an effective horizontal scanning duration T.sub.VH, sampling frequency fs becomes equal to the number of pixels per horizontal scanning line divided by the effective horizontal scanning duration, that is, X/T.sub.VH.
The printing of one frame is achieved by successively printing one line in the vertical direction of the pixel array from left to right. First, in the first frame period, the pixels of the first vertical line denoted by x's at the left are consecutively sampled from top to bottom, thereby being written in first line memory 16a. In the second frame period, the pixels of the second vertical line are sampled to be written in second line memory 16b and, at the same time, the pixels of the first vertical line written in first line memory 16a are read out to be printed via IGC 19 and TPH 20. Then, in the third frame period, the above-.described operation is reversely performed, so that the writing in first line memory 16a and the reading out of second line memory 16b are carried out. By doing so, the printing of 600 vertical lines is completed which results in the printing of one color. The still picture of one frame is printed by repeating the above printing operation for each of the three colors.
FIG. 3 is a timing chart showing read/write operations of line memory. During the first frame period, the first vertical line is written in first line memory 16a. During the second frame period, the pixels written in first line memory 16a are read out during the odd-field period, thereby being printed. The even-field period is provided as a heat-emission period of the TPH. During the third frame period, the first vertical line is written in first line memory 16a.
Meanwhile, the second vertical line is written in second line memory 16b during the second frame period. In the third frame period, the pixels written in second line memory 16b are read out during the odd-field period, thereby being printed. The even-field period is provided as the heat-emission period of the TPH. During the fourth frame period, the fourth vertical line will be written in second line memory 16b.
In the above-described color video printer, given the frame period of the NTSC system as 1/30th of a second (.apprxeq.33 ms), the time required for printing one color of the still picture of one frame can be calculated thus: 33 ms.times.600 lines=19.8 s, so that the time required for printing one frame is 19.8 S.times.3=59.3 S. Therefore, if the time for paper-feeding and paper-discharge takes twenty seconds, the total printing time for the still picture of one frame becomes approximately 80 seconds.
Experimentation shows that picture quality remains unaffected even though the printing is successively performed by eliminating the heat-emission period illustrated in FIG. 3. Accordingly, it is preferable to provide an apparatus capable of increasing printing speed by successively printing two vertical lines during one frame period.